The present invention relates to the laboratory preparation of dental prostheses for installation on implant fixtures previously installed in the mouth of the patient. More particularly, the invention relates to the construction of a working model and the analog elements employed therein for use by a dental technician in the fabrication of restorative dental prostheses.
Dental prostheses which are fixedly attached to the jaw of a patient through the use of dental implants are common. Such prostheses employ implant fixtures which are permanently fused to the jaw of the patient, and crowns comprising abutments upon which a prostheses is formed for ultimate attachment to the implant fixture. Typically, a two-stage procedure is employed. In the first stage, an implant fixture is installed in a receptacle formed in the bone of the jaw and the fixture allowed to become bonded to the bone by osseointegration occurring over a period of several months. During the second stage of the procedure, the gum that overlies the now-fused implant fixture is incised to expose the top end of the fixture and a previously prepared crown is attached thereto by fitting the crown abutment to an abutment-receiving portion of the fixture and connecting the crown by means of a suitable screw that cooperates with a threaded socket in the implant fixture.
The description herein is directed principally to prostheses of the so-called "UCLA"-type in which the abutment is integrally-formed with the prosthesis casting. It should be understood, however, that the invention is equally applicable to standard abutment-cylinder assemblies in which the abutment is a separately formed element designed for threaded attachment to the fixture, as hereinafter described.
In preparing a dental prosthesis or crown for attachment to an implant fixture, a work holder, called a "working model", in the form of an accurate replica of the existing tooth and gum structure of the patient is made from molding material, such as die stone or plaster-of-paris. Such working model is utilized by the technician during the construction of the crown, which normally comprises a hard core or "abutment" that may be formed of metal, ceramic, plastic, or other hard material, and covered by a ceramic material, usually porcelain, or by plastic or composite materials that replicate the tooth or tooth group to be replaced. (In the description herein, the crown consists of two lower central teeth. The invention, however, contemplates crowns for replacing a single tooth, or groups of teeth, for the described or other regions of a patient's mouth.)
In essence, the working model is constructed by making an impression of the teeth and gums in the concerned region of the patient's mouth by enclosing them in dental impression material, as for example, silicone, or other formable rubber-like materials, to form a mold or "negative" into which molding material, such as plaster-of-paris, is placed and permitted to harden. The solidified model forms a work holder, sometimes termed "a replica cast", which duplicates the affected region of the patient's dentition and within which the crown may be inserted during preparation in order to accurately determine its relationship to adjacent teeth for fitting purposes.
Presently, it is the practice in producing working models of the concerned type to make a single casting of molding material duplicating the patient's dentition and in which a laboratory analog is fixedly installed at the proper location. The upper end of the analog contains an internally threaded socket and an abutment-receiving portion that, in cooperation with the socket, enables a crown intended for ultimate attachment to an implant fixture to be set in the working model. Attachment of the crown to the laboratory analog is effected by passing a headed screw through an open-ended axial hole extending through the crown abutment and connecting it to an internally threaded socket in the analog. Typically, the screw employed for connecting the crown to the analog contains a slotted head for reception of a screw-driving tool and is structurally similar to the screw employed for connecting the crown to the patient's implant fixture.
Thus, in preparing the crown, during the process of which the crown must be repeatedly removed and re-installed in place in the working model, it is necessary that the technician employ the screw-driving tool to release the screw upon each removal of the crown from the working model and, conversely, to install the screw upon each mounting of the crown in place in the working model. This repeated manipulation of the screw each time the crown is removed or reinstalled on the working model is laborious and time-consuming and consequently adds substantially to the time and effort required for preparation of the crown. Furthermore, it increases the danger of eroding or otherwise damaging the cast working model.
It is, therefore, to the solution of this problem that the present invention is directed.